In order to resolve densely used frequency resources along with an increased data communication volume in the cellular system, as a technology to realize a high frequency usage efficiency and high-speed transmission, downlink MIMO (Multiple-Input Multiple-Output) transmission in which a plurality of transmitting signals are subjected to spatial multiplexing using a plurality of transmitting antennas included in a base station has been actively researched. In the downlink MIMO transmission, Single User-MIMO (SU-MIMO) in which a plurality of transmitting signals are subjected to spatial multiplexing which are addressed to a single terminal having a plurality of antennas is an essential technology to improve maximum transmission speed of each terminal, in which, however, there is a possibility that a transmitting antenna included in a base station is not effectively used due to limitation of the number of antennas included in the terminal. Contrary to this, as shown in FIG. 7, Multi User-MIMO (MU-MIMO) in which transmitting signals addressed to a plurality of terminals 2000a, b and c are subjected to spatial multiplexing to be transmitted simultaneously enables to effectively use an antenna on a base station side even when only a small number of antennas are included in the respective terminals 2000-a, b and c, thus attracting attention as a technology to improve cell throughput.
In the MU-MIMO transmission, for transmission of signals addressed to a plurality of terminals with a same resource, it is necessary to apply precoding to signals on a base station side in advance for transmission so that those received by respective terminals do not interfere with each other. A method of the precoding is roughly classified into linear precoding in which a plurality of transmitting signals are multiplied by a linear weight, and nonlinear precoding in which a known interference signal is sequentially subtracted from a transmitting signal and thereafter multiplied by a linear weight, in which the linear precoding is, though characteristics thereof are slightly degraded compared with the nonlinear precoding, able to realize spatial multiplexing to a plurality of signals with very simple processing.
The linear precoding includes some types, in which linear MMSE (Minimum Mean Square Error) precoding which minimizes a mean square error between a signal received at each terminal and a desired signal is a method in which excellent transmitting characteristics are obtained with relatively simple processing. Here, a weight PMMSE according to linear MMSE is shown as follows in a case where a base station having four transmitting antennas performs spatial multiplexing to signals for four terminals each having two receiving antennas. Note that, each terminal is assumed to notify a base station of channel states between one of the two receiving antennas included in each terminal and the four antennas included in the base station. When a matrix which collectively represents channels notified from each of the terminals respectively is assumed to be H, the weight PMMSE is expressed as follows.[Math 1]PMMSE=(HHH+ξI4×4)−1HH  (1)
Wherein, H=[H11T H21T H31T H41T]T, and Hmn is a vector with one row and four columns representing channels between an antenna n of a terminal m and four antennas in a base station. Further, ξ is a mean noise power to signal power ratio, and Ik×k shows a unit matrix of k rows and k columns. Here, when a signal vector addressed to terminals is assumed to x=[x1 x2 x3 x4]T, a receiving signal vector of collected signals received by respective antennas for notification of a channel to the base station, y=[y11 y21 y31 y41]T is represented by y=HPMMSE x+n. Wherein n=[n11 n21 n31 n41]T is a vector representing thermal noise added at the antenna n of the terminal m.
Multiplying a signal addressed to each terminal by such a linear weight PMMSE for transmission enables to perform spatial multiplexing to signals addressed to a plurality of terminals while suppressing multi-user interference in which signals addressed to respective terminals interfere with each other on a receiving side. In this manner, the terminal m for receiving signals after being subjected to spatial multiplexing in this manner demodulates signals received by an antenna 1 respectively to be able to obtain desired information, however, each terminal has two receiving antennas respectively and even at an antenna 2 which is not a target of spatial multiplexing processing in the base station, some signal is to be received. The signal received by the antenna 2 is not the target of spatial multiplexing processing in the base station, and thus includes a considerably large amount of multi-user interference, so that even when the signal received by the antenna 2 of each terminal is demodulated, it is impossible to obtain a desired signal correctly. However, it has been known that since this signal also includes a desired signal component, appropriately combined with the signal received by the antenna 1 and the combined signal is demodulated, excellent receiving characteristics are thereby able to be obtained compared to a case where a signal received only by the antenna 1 is demodulated (Non-Patent Literature 1).
Here, a receiving signal vector ym=[ym1 ym2]T received by two antennas of the terminal m is expressed by ym=HmPMMSE x+nm. Wherein Hm=[Hm1T Hm2T]T, and nm=[nm1 nm2]T is a thermal noise vector at the terminal m. In a combining method shown in Non-Patent Literature 1, such the received signal is multiplied by a receiving MMSE weight expressed by the following formula and subjected to combining. The receiving weight is to minimize a mean square error between a signal received at each terminal and a transmitting signal.[Math 2]Wm=(HmmPMMSE)H{HmPMMSE(HmPMMSE)H+ξI2×2}−1  (2)
The signal after multiplication of a receiving weight at the terminal m is expressed as Wmym=Wm (HmPMMSE×+nm), and an m-th signal among the signals after multiplication of the receiving weight is a desired signal of the terminal m. At each terminal, signals received by two antennas are combined in this way, so that it becomes possible to combine a desired signal component included in a signal received by an antenna which is not subjected to spatial multiplexing on a base station side, thereby allowing to obtain excellent receiving characteristics compared to the case of demodulating a signal received by one antenna.
Additionally, in general, there is a difference between the time of measuring a channel H at a terminal and the time of spatial multiplexing transmission using a linear weight based on the measured channel H, and thus, in the case of movement of a terminal, or the like, the channel varies temporally so that the multi-user interference is not completely suppressed, which poses a problem, however, since the combining method shown in Non-Patent Literature 1 is a method of combining signals received by two antennas based on an actual channel state in spatial multiplexing transmission, which is considered to be effective for suppressing multi-user interference occurred due to such time variation of a channel.